The need for some form of power control circuitry is ubiquitous in electronics. Many power sources are available to electronic system designers. Multiple energy or power sources are often accessible to some degree, however, their availability may occur intermittently and in various combinations, making capitalizing on their availability problematic. Examples of potential power sources that may be capable of using energy available in a system's operating environment include; piezoelectric generators, solar generators such as photovoltaic circuits, thermoelectric “Seebeck” generators, wind or other mechanical generators, conventional AC sources, disposable batteries, and so forth. Such sources may or may not be continuously available, due to the nature of the energy source, or due to changes in the operating environment of the electronic system. Therefore, an intermediate storage element such as a rechargeable battery or capacitor is often used in order to provide continuity of supply. For example, energy is captured from an available source, e.g., vibration energy is harnessed using a piezoelectric generator, and is stored, e.g., with a battery or array of batteries, for ultimate use by an electrical load. Thus, circuitry useful for charging storage elements such as capacitors or batteries is an important consideration in the design of electronic systems, and particularly for portable systems. In addition to the variability of energy sources, the output needs encountered by a given electronic system may also be variable. Examining batteries as a common example of power control, it is well known that restoring a discharged battery to a fully charged state, and maintaining it in a fully charged condition, is a multifaceted problem involving a number of factors. For example, battery chemistry, (e.g., Nickel-Cadmium (Ni—Cd), Nickel Metal-Hydride (NiMH), and Lithium-Ion (Li-ion)), battery age, useful life, physical environment, capacity, and number of cells, are just some of the factors that must be considered in selecting not only a suitable battery, but also a power control circuit to optimally utilize the battery. The availability of charging power is one such consideration that must be addressed in charging system and associated power control design. Useable apparatus and systems for harnessing variable and intermittent input power levels for power control circuits and electronic systems would provide useful advantages in the arts.
It is known that it is often necessary to provide selectable charging level controls for regulating the output of charging circuitry. Charging circuits known in the arts are generally designed for accepting a fixed input power level, and for selecting from two or more predetermined output power levels in order to charge a storage element. For example, in some applications, such as a “universal” charger, it is desirable to accommodate different capacity batteries by providing different pre-determined charging output levels. Appropriate charging rates are generally dependent upon battery chemistry and construction. Generally, fast charging refers to methods that can charge a storage element in one to two hours, and slow charging refers not only to longer charging periods, but also implies a charging level low enough that overcharging the battery is less of a potential problem. It is known in the arts to provide selectable pre-determined charging levels based on a scheme for fast charging a battery (or other storage element) up to a set level, and then providing a lesser slow charging current for maintaining the storage element in a fully charged state. Such charging schemes typically rely on some form of temperature or voltage sensing, and perhaps a timer, in order to protect against overcharging, which could result in shortening battery life, battery failure, or a spectacular explosion. At the other end of the charging continuum, for most battery types, once the battery is discharged into an undervoltage, or overdischarged, condition, a continuing voltage or current draw from the battery beyond the undervoltage level could chemically degrade the battery, permanently reducing its charge capacity, reliability, and useful service life. Thus, particularly for power control systems used with batteries as storage elements, charging circuit efficiency, reverse-current protection, and low quiescent current, are highly desirable traits.
The present invention is directed to overcoming or diminishing problems present in electronics, power control circuitry, and particularly charging systems, of the prior art, and contributes one or more heretofore unforeseen useful advantages to the arts.